Field of the Invention
The present disclosure relates to an exhaust gas cleaning catalyst.
Discussion of the Background
Heretofore, in order to clean an exhaust gas of an internal combustion engine, exhaust gas cleaning catalysts, such as a three-way catalyst, have been used. The three-way catalyst removes monoxide carbon (CO) and hydrocarbons (HC) contained in an exhaust gas by oxidation thereof and also nitrogen oxides (NOx) contained in the exhaust gas by reduction thereof. As the three-way catalyst described above, for example, a catalyst has been widely used which includes a heat resistant honeycomb base made of cordierite or the like, a support layer of γ-Al2O3 or the like provided on the honeycomb base, and a noble metal, such as platinum (Pt) or rhodium (Rh), supported on the support layer.
As the support, since a large specific surface area and high heat resistance are required, besides Al2O3, for example, SiO2, ZrO2, and TiO2 have been generally used as the supports. In addition, in order to reduce the change in exhaust gas atmosphere, in addition to the above support, CeO2 having an oxygen storage capacity (hereinafter referred to as “OSC”) has also been used in combination. Furthermore, in order to improve the OSC durability of CeO2, for example, a composite oxide of CeO2 and ZrO2 has been used.
In addition, because of recent strengthened regulations on exhaust gas, it has been strongly required to clean an exhaust gas immediately after an engine is started. Hence, there has been desired the development of an exhaust gas cleaning catalyst which can be activated in a lower temperature zone and which can exhibit high catalytic activity from a low temperature zone immediately after an engine is started.
Hence, an exhaust gas cleaning catalyst has been proposed which contains a cerium-containing oxide as a primary component and palladium and zinc, each of which is supported on the cerium-containing oxide (Japanese Unexamined Patent Application Publication No. 2009-241057). According to this exhaust gas cleaning catalyst, although the content of the noble metal is small as compared to that of a related exhaust gas cleaning catalyst, high durability can be achieved, and excellent catalytic activity can also be obtained even in a low temperature zone.